It is with keen pleasure and considerable nostalgia that I once again pick up my copy of the Treatise to begin these first segments of the tutorial on the philosophy of David Hume. I first engaged deeply with Hume's thought sixty years ago as a seventeen year old Harvard sophomore, in a course taught by Henry David Aiken and devoted entirely to the Treatise. In my Autobiography, I have told the story of my rather contentious relationship with Aiken [see Volume One, Chapter Two], but eventually he and I were colleagues for three years, and he even invited me to his home for dinner -- a rarity in the socially dysfunctional Harvard Philosophy Department of those years. Yesterday, I was idly Googling myself [let him who is without sin cast the first stone!], and discovered to my astonishment that a rare book seller is offering an "original" copy of my 1960 journal article on Hume, inscribed by me to Aiken, for a mere 120 Euros. Aiken's library must have been sold after he passed away, and somehow that offprint found its way onto the market. When the article appeared, I ordered a hundred offprints, and half a century and more later, I still had about eighty-five of them. When Susie and I sold our Western Massachusetts house and moved to Chapel Hill, I threw out all but three or four of them, thereby, I suppose, inflating their value through enforced scarcity.
In approaching the argument of Book One of the Treatise, it is useful to see Hume as influenced by two major movements in early eighteenth century thought: the so-called "new way of ideas" of John Locke and the dramatic successes in physics culminating in Newton's Principia. Locke's new way of ideas was roughly what I called, in my recently completed introduction to Kant's Critique, the "epistemological turn," which is to say the dissolving of all philosophical questions into an analysis of the contents, cognitive powers, and limitations of the human mind. The triumph of Newton's Principia was in mathematically deriving Galileo's laws of terrestrial motion and Kepler's laws of planetary motion from a single simple set of very general physical laws, thereby decisively refuting the age-old teaching that the heavens and the earth are composed of entirely different sorts of matter and obey entirely different laws. [For those of you who are not familiar with this tradition, the doctrine descending from Aristotle and his Greek predecessors had it that there are four terrestrial substances -- air, earth, fire, and water -- and a fifth substance, or quintessence, finer and purer than the terrestrial substances, of which the heavenly bodies and the spheres in which they are embedded are composed.]
A central point of dispute in the new way of ideas concerned the source or origin of our mental contents -- our "ideas," as Locke called them. Following the line of argument advanced by Plato in the Phaedo and in other dialogues, the Continental Rationalists argued that certain key ideas -- the mathematical ideas, the idea of God, and metaphysical ideas such as substance and accident -- must be innate in the human mind, inasmuch as they could not have been abstracted from sense experience. In opposition to this thesis, Hume embraced wholeheartedly the Empiricist insistence that all of our perceptions derive from sense experience, an insistence that serves as the starting point for both his sceptical and his constructive doctrines. [Hume uses "perception" as Locke used "idea" and as Kant after him used "representation," to mean roughly "mental content," or perhaps, more precisely, "cognitively significant mental content."]
From Newton, Hume took the idea that a single force -- gravitational attraction in the case of physical bodies -- could if properly understood serve to explain the motions of bodies in space. As we shall see, Hume thought he had found the analogue to gravity in the sphere of the mental, and he clearly hoped that he could carry that notion in Moral Philosophy as far as Newton had in Natural Philosophy. Quite the most interesting thing in all of Hume's philosophy, we shall see presently, is the reason why this dream precisely could not be realized.
Hume announces his embrace of the new way in the Introduction to the Treatise. [A warning -- Hume wrote so beautifully, with such concision, clarity, precision, and elegance, that the temptation is overwhelming simply to incorporate large chunks of the Treatise into this tutorial. Any student of philosophy who imagines that it is necessary, or even desirable, to write turgidly and obscurely when engaging with deep questions would do well to spend a long time reading Hume and striving to emulate him.] This passage is from the fifth paragraph of the Introduction:
" 'Tis evident, that all the sciences have a relation, greater or less, to human nature; and that however wide any of them seem to run from it, they will return back by one passage or another. Even Mathematics, Natural Philosophy, and Natural Religion, are in some measure dependent on the science of MAN; since they lie under the cognizance of men, and are judged of by their powers and faculties. 'Tis impossible to tell what changes and improvements we might make in these sciences were we thoroughly acquainted with the extent and force of human understanding, and cou'd explain the nature of the ideas we employ, and of the operations we perform in our reasonings."
The extent and force of human understanding, the nature of the ideas we employ, and the operations we perform in our reasonings -- these, with great concision, delineate Hume's subject in the Treatise. A word of explanation of the term "natural religion." It was common in the eighteenth century to distinguish between religious truths accessible only to those favored with God's direct word, and religious truths available to any person merely by the use of his or her powers of reason. The first were known as "revealed religion," the second as "natural religion." One of the central questions of Western philosophy for the eighteen centuries separating the Incarnation from the publication of the Treatise and the First Critique was the relation between the two.
One further observation that may be of interest to readers before I open the Treatise to page 1 and begin. Hume and his fellow empiricists engendered almost two centuries of what came to be called "sense datum epistemology." But there were a number of difficulties with this way of proceeding that led English philosophers and others in the early twentieth century to turn their attention away from sense contents and toward the language, the words, that we use to describe our sense experience. This came to be called "the linguistic turn," analogously to the "epistemological turn" of a century and a half earlier. Initially, this turn led to a good deal of discussion of "private languages" used to describe subjective and unshareable sense experience, but Wittgenstein and others argued persuasively that language is inherently public and intersubjective, thereby finessing the vexing problem of "other minds." Well, so much for a stroll down memory lane.
"All the perceptions of the human mind resolve themselves into two distinct kinds, which I shall call IMPRESSIONS and ideas. The difference betwixt these consists in the degrees of force and liveliness with which they strike upon the mind. and make their way into our thought or consciousness." Thus the Treatise begins. Right away, the alert reader will feel a certain concern, for whereas impressions [which is to say, colors, sounds, tastes, and so forth, along with passions and emotions such as love, hatred, anger, fear, pleasure, and desire] may seem appropriately described as mere mental contents, or contents of consciousness, ideas, as we customarily understand them, seem to have a referential or intentional structure -- they are, typically, ideas of something. Is that really just a difference in "force and liveliness?" We shall come back to this question in a bit.
Immediately, Hume claims that it is the impressions that are the original contents of our consciousness. Ideas are merely "faint images" of our impressions, or copies of them. Thus is born the so-called "copy theory of ideas." This claim is enormously powerful, because it allows Hume to ask, over and over again with regard to such philosophically problematic ideas as the idea of causal necessity, or the idea of substance, or the idea of God, where the impression is of which the idea in question is a copy. Indeed, this might be called the organizing rhetorical trope of Book One of the Treatise.
Now, Hume introduces two distinctions, one seemingly easy and unproblematic, the other of the greatest importance for his philosophical undertaking. Easy first: some of our impressions are simple, and cannot be divided or separated into parts. A minimally discriminable red patch is an example. Others are complex, such as the variegated expanse of a visual field. The same distinction can be drawn between simple and complex ideas. Every simple idea, Hume insists, is a copy of a simple impression. There are no simple ideas for which there is no corresponding simple impression. But there are many complex ideas that are not copies of any identifiable complex impression, for such ideas are constructed by the mind from the bits and pieces of previous experience, only in new arrangements. So the idea of a Centaur is an idea of a creature with the head and torso of a man [of which we have preceding impressions] and the body of a horse [likewise.] All fictions are generated by the mind's imaginative power to reassemble its simple ideas in new complexes and arrangements.
The other distinction introduced by Hume is between what he calls Impressions of Sensation and Impressions of Reflexion. The latter impressions -- lively and vivacious -- are just like the impressions of sensation, but their origin is different. They do not arise in our minds immediately upon the sense organs being affected by external objects. Instead, there is a two-step process by which they come into being. First, I perceive some object or state of affairs -- for example, the sight of my neighbor dumping his trash on my lawn. I reflect on what I have seen, and there is triggered in me by this act of reflexion a natural internal response, resulting in a new impression of great force and vivacity arising in my consciousness, viz., anger. The passions, Hume asserts, are all impressions of reflexion. What is more, as we shall see, so are our moral feelings, and so most especially is the impression of the necessity of connexion betwixt cause and effect on which Causal Judgments are founded.